"since" + present perfect

I've just heard the following line in a song:

It's only been three days since I have seen your face.

How do you feel about this sentence? Shouldn't it say "It's only been three days since I _saw_ your face?"

I've studied that it is right to say:
I've known my best friend since 10 years.

No, I've known my friend for 10 years.

It seems that I've forgotten everything since I've studied english.
but is it possible to say my sentence.
This grammar is hard cheese for me. I have no sence for grammar.
Just write the words - they comes without thinking of any rules. I have no longer practice. It's boring.

Tom,

The sentence seems okay, but I'd say your sentence flows better and sounds more natural. It's certainly what I would say.